In general, ceramics have excellent mechanical and electrically insulating properties as well as excellent high-temperature characteristics, and they have attracted attention as materials for use as parts of equipment for manufacturing semiconductors. Due to a large amount of shrinkage during sintering to produce a ceramic, such parts must be manufactured from a sintered body by machining such as grinding in order to provide a desired shape and dimensions with high accuracy. However, most ceramics are difficult to machine.
Machinability of a ceramic can be improved by incorporating a cleavable ceramic component such as mica or boron nitride which is dispersed in the ceramic or, in the case of a crystallized glass ceramic, in its glass matrix. This type of ceramics are generally referred to as machinable ceramics. In view of their good machinability, they are often used to manufacture some parts of semiconductor inspection equipment, which must have good insulating properties and be capable of undergoing fine machining with high precision.
However, there are few materials which have the combination of high strength and excellent machinability required for high-precision fine machining along with a coefficient of thermal expansion close to that of silicon. In addition, the color of conventional machinable ceramics is not uniform, and thus they have a poor appearance which reduces the value of a commercial product made from the materials. Moreover, these ceramics have a white or pale color with high reflectivity, which prevents accurate inspection and measurement by image processing of machined parts formed from the ceramics.
The electrical characteristics of semiconductor elements such IC's and LSI's are typically inspected using a probe card, which has a large number of probes for inspection. Inspection is carried out by bringing all the probes of the probe card into contact with the electrode pads of a semiconductor element simultaneously.
FIG. 1a is a schematic vertical cross-sectional view of a probe card having inspecting probes for use in inspecting a semiconductor element. A probe card 1 formed from an insulating material such as a ceramic has at approximately its center an opening 10 having dimensions which are nearly the same or larger than those of a semiconductor element to be inspected. The opening 10 is normally flared upwards as illustrated. Inspecting probes 2 which are equal in number to the electrode pads formed on the semiconductor element are secured to the top surface of the probe card 1 by an adhesive, for example.
The inspecting probes 2 are normally made of a conducting metallic material. The tips of the probes 2, which are bent into roughly the shape of an L, slightly protrude from the lower surface of the probe card 1 through the opening 10 so as to arranged with the same pattern as the electrode pads of the semiconductor element. Although not illustrated, the other ends of the probes 2 are electrically connected by solder or the like to an electrically conducting circuit formed on the top surface of the probe card 1. In order to prevent the probes 2 from contacting each other, the periphery of each probe 2 except for its tip may be covered with a heat resisting resin or similar material.
The electrical properties of a semiconductor element (not shown) are inspected by placing the probe card 1 atop the semiconductor element to be inspected and pressing down the probe card 1 so that the tips of the inspecting probes 2 protruding through the opening 10 contact the electrode pads of the semiconductor element. In order to accurately perform inspection, it is essential for all of the large number of inspecting probes to simultaneously contact the electrode pads of the semiconductor element disposed beneath it with certainty. However, the probes are normally made of a slender metallic material, so they easily bend when the probe card 1 is pressed downwards, and due to the bending, it is easy for the positions of the tips of the probes 2 to slip. As a result, it is difficult for the probes 2 to contact the electrode pads with certainty.
As shown in FIG. 1b, in order to make it easy to accurately align the inspecting probes 2 with the electrode pads of a semiconductor element, a probe guide 3 formed from an insulating plate can be fitted into the opening 10 of the probe card 1 so as to block the opening 10. The probe guide 3 has through holes 12, through which the probes 2 pass so that the tips of the probes 2 project from the lower surface of the probe guide 3. The through holes 12 are arranged with the same pattern as the electrode pads. The probe guide 3 serves to limit lateral movement of the probes 2 due to bending and allows the probes 2 to contact the electrode pads more accurately.
The through holes 12, which have a somewhat larger diameter than the inspecting probes 2, are formed in the probe guide 3 with the same pitch as the electrode pads. In recent LSI's, which are achieving significantly higher mounting densities, it is not unusual for the pitch of electrode pads to be 100 micrometers or smaller.
For example, as shown in a plan view and a cross-sectional view in FIG. 1c and FIG. 1d, respectively, when the pitch of electrode pads is 70 micrometers, if the diameter of each through hole 12 is 60 micrometers, the wall thickness between adjoining through holes (the minimum distance between holes) becomes an extremely small value of 10 micrometers. Thus, it is necessary to form such small-diameter, thin-walled through holes must be formed with high accuracy in a probe guide by means of drilling, for example.
A different type of a probe guide is shown in a perspective view in FIG. 2. In FIG. 2, a frame-shaped probe guide 3a, which may be either an integral insulating part or assembled from insulating panels, have vertical slits 14 on at least one side member, usually two or four side members, of the frame with the same pitch as the electrode pads of a semiconductor element to be inspected (not shown). The probe guide 3a may be fitted into the opening of a probe card (not shown) by inserting it into the opening of the probe card from below so that each probe 2 of the probe card extends through the corresponding slit 14. Again, lateral movement of the probes 2 is limited by the slits of the probe guide, and they can be made to more accurately contact the electrode pads.
FIG. 3 is a schematic cross section of one side member of a frame-shaped probe guide 3a having slits 14 as shown in FIG. 2. As illustrated, the shape of slits 14 are generally defined by the depth and width of each slit and the wall thickness which is the distance between adjacent slits. The slits of a probe guide are usually deep and fine with a thin wall thickness. For example, as shown in FIG. 3, slits 14 may have a depth of 300 micrometers and a width of 40 micrometers with a wall thickness of 15 micrometers. Such slits are generally formed by grinding using a grindstone and/or grinding wheel.
Naturally, a probe guide needs to be electrically insulating in order to prevent short circuits between inspecting probes. It is necessary for its volume resistivity to be at least 1.0×1010 Ω·cm.
Conventional probe guides were made of plastic or a machinable crystallized glass ceramic comprising Al2O3, SiO2, and K2O. In recent years, they may be made from a boron nitride-containing machinable ceramic.
A plastic probe guide generally cannot be used for inspection at a high temperature. In addition, it is not possible to achieve sufficient dimensional accuracy of through holes or slits to inspect semiconductor elements of higher densities with smaller pitches of electrode pads.
A probe guide made from a crystallized glass ceramic is capable of performing high temperature inspection. However, the coefficient of thermal expansion of a crystallized glass ceramic is large compared to that of a semiconductor element, and depending upon the inspection temperature, positional deviation may occur between inspecting probes and the electrode pads of a semiconductor element being inspected.
In addition, the strength of a crystallized glass ceramic is not so high that chipping and cracking easily take place during drilling or other machining, and an adequate dimensional accuracy cannot be obtained. See JP-A 58-165056 (1983).
Furthermore, a conventional crystallized glass ceramic is white. Therefore, when image processing is carried out in order to inspect the dimensions of tiny through holes or slits formed in a probe guide or to perform positioning of a probe guide on a probe card, light is easily reflected from the ceramic surface of the probe guide, thereby making it difficult to obtain an accurate image. In addition, due to the white color, dirt on exterior of the ceramic easily stands out and decreases the value of a product manufactured from the ceramic. See JP-A 58-165056 (1983).
A composite ceramic material comprising aluminum nitride and boron nitride has a coefficient of thermal expansion which is close to that of silicon. Therefore, when using a probe guide made of such a material, positional deviations caused by thermal expansion are small. However, the machinability of this material is poor, so it is not suitable for high precision fine machining. In addition, it has an irregular coloring which reduces the value of a product made from it. See JP-A 60-195059 (1985).
A high-strength machinable silicon nitride/boron nitride composite ceramic material has been proposed, but its coefficient of thermal expansion is small compared to that of silicon. Thus, when it is used in a jig for inspecting semiconductors such as a probe guide, depending upon the inspection temperature, positional deviations easily occur. See JP-A 2000-327402.
Accordingly, there still remains a need for a high-strength machinable ceramic capable of being processed by fine machining with high precision and which can be uniformly colored so as to have low optical reflectivity and has a coefficient of thermal expansion which is close to that of silicon.